Lithography is a key factor in the drive for higher levels of micro-circuit integration. Dynamic RAMs have quadrupled in the level of integration every three years as a result of the reduction in minimum geometries and increases in chip size. As minimum geometries approach 0.5 .mu.m and below, lithography alternatives include optics, electron beam direct write, X-ray and electron/ion beam proximity technologies. The latter three technologies are still in their infancy relative to optical lithography and still have obstacles to overcome, including decreased throughput, low source brightness and mask complexity, respectively.
While optical lithography continues to be the dominant technology because it is well established and is capable of implementing sub-micron resolution at least as low as 0.35 .mu.m, efforts into attaining smaller geometries are looking toward the newer technologies. In both optical lithography and its alternatives, progress into the realm of shorter wavelengths introduces increased sensitivities to minute surface imperfections including contaminants on optical surfaces, aberrations introduced by lenses and mirrors, photoresist thickness and surface variations and wafer flatness and planarity.
Addressing the issue of wafer flatness in particular, ideally, perfect flatness could be maintained at the patterning steps of the process by using the proper vacuum chuck. The wafer chuck of a stepper is commonly mounted on air bearings with precision x-y control for fine adjustment and smooth motion. The surface of the wafer chuck is generally presumed flat and movement of the chuck is tightly controlled to prevent height variations with lateral motion. However, assuming the wafer chuck and its motion are perfectly flat, there is no assurance that the wafer or photoresist layer themselves are flat in a scan from one point of the wafer to another. While wafer flatness can be a cause of a focus problem at any exposure wavelength, the problem becomes particularly pronounced in the sub-micron range where the amount of deformation approaches both the exposure wavelength and minimum dimension. Also, as larger fields are utilized, processing many die in one field, it may even be desirable to deform a perfectly flat wafer to provide an acceptable depth of focus for very large fields.
A small amount of wafer warpage can be expected due to repeated high temperature steps during processing and the compressive strains introduced by patterned features having different coefficients of expansion. Other factors in susceptibility to warpage are wafer thickness and orientation. Further, feature step heights in non-planarized processes can cause surface irregularities within the photoresist resulting in variations in exposure from the top to the bottom of a step.
It is known that wavefront aberrations can be corrected by introducing the phase conjugate of the aberration into an optical element such as a lens or mirror. For example, chromatic variations can be corrected by selected refractive index differences in a lens.
The correction of aberrated wavefronts reflected from a mirrored surface and the addition of known distortions to laser signals is known and has particularly been used in ground-based telescopes where aberrations are caused, for example, by thermal gradients, atmospheric turbulence, and the optical system. Selective local deformation of a mirror's reflecting surface may be achieved by the use of distortive actuators which are selectively energized by the application of electrical signals thereto to produce mechanical forces upon the rear surface of the mirror. Precise control of the distortions introduced into the mirror's reflecting surface may be achieved by spacing the actuators close to each other and by having the surface area of the mirror influenced by each actuator being kept as small as possible, and by making the structure which carries the reflecting surface as flexible as possible. Such a mirror may consist of a single thin sheet of reflective material as disclosed in U.S. Pat. No. 4,655,563 or may be a segmented mirror as disclosed in U.S. Pat. No. 4,944,580.
It is further known that wafers, in spite of their apparent brittleness, are capable of limited flex and deformation, similar to a thin sheet of glass. If this characteristic were not present, wafers would break under the differential strains produced by selective deposition and removal of thin films on the wafer and the high temperature processing of those films.
It would be desirable to provide a wafer chuck for use in a lithography system which is capable of selectively deforming portions of a wafer and taking advantage of a wafer's flexibility to compensate for warpage of the unprocessed wafer and warpage induced during processing, thus permitting highly accurate level-to-level alignment and providing better control of uniformity of dimensions of the projected pattern. It is to such a device that the present invention is directed.